A projection exposure apparatus configured to transfer a pattern image of a reticle as a mask onto each shot area on a resist-coated wafer (or a glass plate and so forth) serving as a photosensitive substrate through a projection optical system is used for manufacturing semiconductor elements and the like. A reduced projection type exposure apparatus (a stepper) applying a step and repeat method has been conventionally used as the projection exposure apparatus in many cases. Meanwhile, a projection exposure apparatus applying a step-and-scan method configured to scan and expose the reticle and the wafer. synchronously is also drawing attentions in these days.
Resolution of the projection optical system incorporated in the projection exposure apparatus becomes higher as an exposure wavelength used therein becomes shorter or as the numerical aperture of the projection optical system becomes larger. Therefore, the exposure wavelength used in the projection exposure apparatus is becoming shorter every year while the numerical aperture of the projection optical system is gradually increasing along developments in finer process rules for manufacturing integrated circuits. Although the dominant exposure wavelength today is 248 nm of a KrF excimer laser, an ArF excimer laser having a shorter exposure wavelength of 193 nm is also being put into practical use recently.
Incidentally, along the reduction in the wavelength of exposure light, types of glass materials having sufficient optical transmittance for obtaining light intensity required for exposure while ensuring a desired image-forming performance are limited. In this context, there is disclosed a projection exposure apparatus of a liquid immersion type configured to fill a space between a lower surface of a projection optical system and a surface of a wafer with a liquid such as water or an organic solvent, and to improve resolution by utilizing a phenomenon that the wavelength of exposure light in liquid becomes 1/n (n denotes a refractive index of liquid usually ranging from about 1.2 to 1.6) times as large as the wavelength in the air (Japanese Patent Application Laid-Open Gazette No. Hei 10-303114 (JP 10-303114 A)).